Any reference to background art herein is not to be construed as an admission that such art constitutes common general knowledge in Australia or elsewhere.
The use of waterproof coatings on a range of substrates is common in a wide range of applications. One example is the waterproofing of paper and paperboard products used in the food packaging and transportation industries. It is important in these applications to protect the food from the ingress of moisture which may affect the quality of the food or even cause spoiling. Waterproof coatings also preserve the packaging's structural integrity in high moisture environments.
Existing waterproof coatings of card and corrugated paper packaging are often made from wax which may provide an effective barrier to water but which are difficult to recycle. Other waterproof coatings are known which contain a range of polymeric hydrophobic components along with fillers and the like. These fillers and other agents are typically required to place the paper substrate into a suitable state for attachment of the actual waterproofing agent. This requires a delicate balance in the formulation whereby pH and temperature conditions are optimal for all of the individual components to exert their effect.
These fillers and other preparatory agents are often less than satisfactory in their performance and overall the coating formulations become complicated in terms of the number of components required to achieve the effect. The presence of these multi-functional components also increases the cost to the consumer.
WO 2000/028141 teaches a composition and method for treating paper products with a ligninosulfonate/kraft lignin blend formulation in a mixture of solvents to improve strength and humidity resistance, described as the preservation of strength in a high humidity environment. The process relies on penetration of the formulation into the paper to bond the fibres together with optional use of sugars to help penetration of the lignin and optional heating. WO 2000/028141 does not provide for water barrier properties even though some incidental improvement in initial water resistance is observed. It also requires very high coating weights of formulation to achieve adequate penetration and has a significant amount of undissolved solids in the formulation which is undesirable.
RU2076125 (Dejneko) generates an oxidised lignin and then employs this as a binder for wood fibres which undergo a hot pressing to form the final particle board. While some water resistance is claimed this process primarily employs lignin as a binder and does not result in a useful water barrier. It also relies on unnecessarily complicated pre-treatment steps for the lignin.
It would be useful to provide for a method of forming a waterproof coating on a substrate, including a paper or a paper product, which is simple, effective and recyclable.